i know

For what it's worth..

Well, you generally don't even -see- any dragons that can't be purchased in a town unless
you're playing on a larger map. If you're playing a "small" map size or "easy" map
difficulty, you generally won't see any outsider more diverse than perhaps Boars or
Halflings.
Now, for the dragons. I know for a fact that you can purchase Crystal Dragons from a
dwelling that is often guarded by a small number of the aforementioned creatures. If you
have a powerful enough army, and a high enough Diplomacy skill, the mineral-typed dragons
may offer to join you, often enough at the cost of a top-level town creature, at up to a
quarter of the number of creatures per week.
Best of luck to you!